1. Field of the Invention
This invention relates to oscillator circuits and methods for measuring capacitance and small changes in capacitance and more specifically relates to an improved oscillator circuit and method for measuring capacitance and small changes in capacitance which eliminates the influence of stray capacitances and linearizes the responses of parallel plate capacitive transducers which are used to sense distance, force, pressure, vibration and other physical parameters.
2. Description of the Prior Art
Capacitive transducers are commonly employed in electronic systems for measuring distance, force and other physical parameters. Many of these transducers take the form of a parallel plate capacitor, in which the capacitance is ideally inversely proportional to the separation of the plates. Measurement of this capacitance indicates the distance between the plates, which may also be related to a force, pressure, temperature or other physical parameter which modulates the plate separation.
The accuracy of circuits and methods for measuring capacitance and small changes in capacitance have typically been limited by the influence of stray capacitances which are unavoidable in any practical embodiment. This problem is particularly acute when measuring capacitances or changes in capacitance that are one picofarad (pF) or less. In these cases, the stray capacitance of the circuit components and the wires connecting the circuit to the transducer may exceed the capacitance that is to be measured. The accuracy of circuits and methods for measuring a parallel plate capacitor have been further limited by the classical fringing effect, in which electric field lines near the edges of the plates deviate from the straight line paths they follow in the ideal case, resulting in a capacitance which is a non-linear function of the inverse of distance. In many applications it is also desirable to provide a digital output signal that is suitable for further processing by digital circuits to provide a display or other indication of the value of the physical parameter that is related to the capacitance being measured.
U.S. Pat. No. 4,347,740 (Townsend) discloses an oscillator circuit for measuring capacitance which is prone to errors induced by stray capacitance and does not provide a means for linearizing the response of a parallel plate capacitive transducer.
U.S. Pat. No. 4,719,409 (Dorman) discloses a system for measuring physical parameters using a capacitive transducer and an oscillator circuit. This system may provide some improvement in the stray capacitance problem, at the cost of added circuit complexity, but does not provide a means for linearizing the response of a parallel plate capacitive transducer.
U.S. Pat. No. 4,737,706 (Eilersen) and U.S. Pat. No. 4,295,091 (Ponkala) both disclose oscillator circuits for measuring capacitance, however, they both do not provide a means for linearizing the non-linear response of a practical parallel plate capacitive transducer.
Therefore, there existed a need to provide an improved oscillator circuit and method for measuring capacitance and small changes in capacitance that provides high performance, is relatively simple, inexpensive to build, immune to stray capacitance, has means for linearizing the non-linear response of a parallel plate capacitive transducer and provides a digital output signal that is suitable for further processing by digital circuits.